[ENH] allow regex for confound specification and impute derivatives

[jdkent]

Summary

Fixes #274.

List of changes proposed in this PR (pull-request)

• allow regex expressions to be passed to -c argument
• test new functionality

[codecov[bot]]

Codecov Report

Merging #273 into master will increase coverage by 0.08%. The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master     #273      +/-   ##
==========================================
+ Coverage    88.8%   88.88%   +0.08%     
==========================================
  Files          10       10              
  Lines         679      684       +5     
  Branches      108      107       -1     
==========================================
+ Hits          603      608       +5     
  Misses         44       44              
  Partials       32       32